The present invention relates to an improved electrical insulation composition. More particularly, the present invention relates to an electrical insulation that may be used effectively in the presence of water. Specifically, the present invention relates to an improved electrical insulation composition that maintains high breakdown strength over time, by minimizing the formation of water trees.
The present invention further relates to a method of preventing the formation of water trees in polymeric electrical insulation composition. The present invention still further relates to an electrical cable containing an insulation composition effective to prevent water trees in the presence of moisture.
Electrical hardware, for example, electric cable, is often used in the presence of or in direct contact with water. When electrical hardware, particularly an electric cable, is operated in the presence of water, the electrical insulation composition used to surround and insulate the hardware is often deteriorated by the water. Thus, medium and high voltage power cables may be subject to dielectric breakdown by a mechanism known in the art as xe2x80x9cwater treeing.xe2x80x9d
In the present description medium voltage power cable refers to a cable which operates at a voltage in the range from about 1 up to about 70 kV and high voltage power cable refers to a cable which operates at a voltage greater than about 70 kV. Medium and high voltage power cables are generally constructed in two basic designs. In the so-called xe2x80x9cdry designxe2x80x9d, a metallic sheath, such as lead, encloses the cable insulation, thereby assuring that the cable insulation always remains in a perfectly dry condition. In the other, called xe2x80x9cwet designxe2x80x9d, a polymeric sheath encloses the cable insulation and, unlike the metal sheath, this polymeric sheath cannot completely prevent diffusion of water, from the outside environment, into the cable insulation layer.
In this xe2x80x9cwet designxe2x80x9d second cable design, when the cable is exposed to a water or moisture containing environment, in the presence of electric stress, a degradation of the electrical breakdown strength is observed over time.
This decay of the electrical properties of the insulation under wet conditions is believed to be due to a phenomenon in the art called xe2x80x9cwater treeingxe2x80x9d.
Water treeing refers to the degradation process of the insulation resulting in the formation of microchannels or tubes having a tree-like appearance. A water tree is initiated at stored areas of water, for examples a defect in the insulation material and may become significant when an electrical field is applied to the electrical insulation. In order to prevent early failure during the operation of the hardware, it becomes necessary to minimize the formation of water trees in the insulator surrounding the electrical hardware.
Prior art insulation materials were often modified by the addition of a material which bonds with water to avoid high local water concentration within the insulation.
A number of additives have been proposed in an attempt to minimize or prevent insulation failure by minimizing and preventing the formation of water trees. A general discussion of this phenomenon and examples of these additives can be found, in for example, U.S. Pat. No. 3,499,791 to Maloney; U.S. Pat. No. 3,956,420 to Kato et al.; U.S. Pat. No. 3,795,646 to MacKenzie Jr.; U.S. Pat. No. 4,206,260 to McMahon; U.S. Pat. No. 4,370,517 to Soma et at.; and U.S. Pat. No. 4,293,459 to Urban et al.
One prior art insulation material, described in U.S. Pat. No. 4,305,849, combines polyethylene glycol with a polyolefin insulating material. Polyethylene glycol suffers from the disadvantage that while it is hydrophilic, making it attractive to water, it has poor compatibility with the insulating polyolefins, such as a polar polyethylene. Because of its poor compatibility, polyethylene glycol is believed to be dispersed as small droplets throughout the polyolefin insulation, these droplets acting as points of attraction for any water diffusing within the insulation. As water accumulates around the additive, areas of high local water concentration may be generated, that can themselves result in defects.
Accordingly, it has been found that, while the addition of polyethylene glycol can delay the onset of water tree formation, over time, it usually results in an overall increase in the number of water trees found in an insulation after a certain period of time.
As noted in U.S. Pat. No. 4,305,849, low hydrophilic, or hydrophobic materials, such as polypropylene glycol, do not prevent the formation of water trees in the insulation.
The present invention provides significant improvements over the prior art compositions by including as an additive within an electrical insulator a material that combines the properties of hydrophilicity, hydrophobicity and mobility within the insulating material.
According to an aspect of the present invention, it has been found that improved water treeing resistance can be obtained by using additives which:
1) possess sufficient hydrophilicity to allow the polyolefin to link water molecules and prevent them from diffusing in the material;
2) possess sufficient compatibility with the polyolefin, to prevent loss or clustering of the additive and maintain diffused water distributed evenly in the mass; and
3) possess a mobility of the hydrophilic portion within the polymeric matrix suitable to allow the hydrophilic portion of the additive to effectively reach and link the diffusing water molecules.
Further advantages of the invention will be set forth in part in the description which follows and in part will be apparent from the description, or may be learned by practice of the invention. The advantages of the invention may be realized and attained by means of the instrumentatities and combinations particularly pointed out in the appended claims.
In a first aspect, the present invention refers to an electrical apparatus having at least a polymeric insulation layer, said layer comprising an electrical insulating composition containing as a major component a polyolefin, characterized in that it comprises an effective amount of a water-tree resistant additive having:
a hydrophilic portion, causing a water absorption in said polymeric insulation layer within a selected absorption range,
a compatible portion with said polyolefin, causing an additive loss by contact with water lower than a selected loss value, and
a mobility of said hydrophilic portion corresponding to a mobility suitable to allow the hydrophilic portion of the additive to effectively reach and link water molecules diffusing in said insulating composition,
such that said polymeric insulation layer has a residual electrical breakdown strength greater than about 35 kV/mm after 120 days accelerated water treeing test according to AEIC CS 5-94.
In a further aspect the present invention concerns an electrically insulating composition containing as a major component a polymer composition selected from the group consisting of polyolefins, characterized in that said composition further contains an effective amount of a polymeric additive having:
a hydrophilic portion, causing a water absorption in said composition within a selected absorption range, and
a compatible portion with said composition, causing an additive loss from said composition, by contact with water, lower than a selected loss value,
said hydrophilic portion having a mobility suitable to allow the hydrophilic portion of the additive to effectively reach and link water molecules diffusing in said insulating composition,
such as to provide an electrical breakdown strength decay in the insulation composition lower than 30% in a 30 days accelerated aging test in water.
In particular, said selected water absorption range is such that the water content of said electrical insulating composition is at least 10% higher than that of the same pure polyolefin with the same aging and increases less than 50% in the period from 100 to 400 hours of exposure to 100% relative humidity at 80xc2x0 C.
In addition, said water absorption is such that the water content of said electrical insulating composition is less than 10,000 ppm after 400 hours of exposure to 100% relative humidity at 80xc2x0 C.
In particular, said additive loss by contact with water is not greater than about 20%. by weight after 120 days accelerated water treeing test according to AEIC CS 5-94. In particular, said mobility corresponds to a contact angle with water of said composition of less than a preselected value; more particularly, said contact angle is less than 75xc2x0 and, preferably, less than 70xc2x0 (according to ASTM D 724-45, using insulating material in the place of the paper).
Said polyolefin is preferably selected from the group consisting of low, medium and high density polyethylene, linear low density polyethylene, ethylene-vinyl acetate copolymer, ethylene-ethyl acrylate copolymer, ethylene-methyl acrylate copolymer, ethylene-ethyl methacrylate copolymer, ethylene-propylene copolymer, ethylene-propylene-diene terpolymer, polypropylene, and mixtures thereof.
In a preferred embodiment the hydrophilic portion is made up of polar units, preferably linear alkene oxide units and, more preferably, ethylene oxide.
In a preferred embodiment the compatible portion is made of aliphatic, aromatic or low polarity units; preferably a hindered alkene oxide, and, more preferably the compatible portion is propylene oxide.
Most preferably, the additive is an ethylene-oxide/propylene-oxide block copolymer, in which the ethylene oxide portion in the additive is from 50 to 80% by weight.
In an alternative embodiment, the compatible portion is a group graftable to said polyolefin and, preferably, an unsaturated aliphatic unit, particularly an allyle containing group.
In another preferred embodiment the compatible portion and the hydrophilic portion are chemically linked to a low polarity group, preferably an amino group, particularly ethylenediamine.
Preferably, the effective amount of water-tree resistant additive is from about 0.1 to about 10% by weight and, most preferably, from about 0.2 to about 0.5% by weight. In a further aspect, the present invention concerns a method for reducing water trees growth in an electrical apparatus having a polymeric insulation layer including a polyolefin as a major component, characterized in that it comprises:
providing an additive in said apparatus;
causing at least a portion of said additive to move and get in contact with water diffusing in said polymeric insulation layer;
causing diffusing water to become linked in an hydrophilic portion of the additive;
causing said additive to link to said polymeric insulation layer.
In a preferred embodiment of the method, causing at least a portion of said additive to move and get in contact with diffusing water comprises providing an additive substantially free to move within said polymeric insulation layer, and causing said additive to link to said polymeric insulation layer comprises providing an additive having an hydrophobic molecular portion.
In an alternative embodiment causing said additive to link to said polymeric insulation layer comprises grafting said additive to said polymeric insulation layer and causing at least a portion of said additive to move and get in contact with diffusing water comprises providing an additive grafted to said polymeric insulation layer and having said hydrophilic portion spayed apart from the graft position.